The present invention relates to vibration dampers, and, more particularly, to damping of vibrations in rotating mechanical structures.
There are a great number of rotating mechanical structures such as engine-driven electrical generator systems, and crankshaft and transmission systems of aircraft, automotive and marine propulsion engines which are often subject to cyclical forces. These cyclical forces cause undesirable torsional oscillatory motions in the rotating structures, and two accepted ways of eliminating them are the coupling thereto of friction damping and vibration absorbing assemblies.
Friction dampers have been described by F. W. Lanchester in U. K. Patent No. 21,139 dated 1910, Den Hartog and J. P. Ormondroyd in "Torsional Vibration Dampers, Transactions of ASME, Vol. 52, p.13 (1930); W. K. Wilson in "Practical Solution of Torsional Vibration Problems", Champan and Hall Ltd., London, Great Britain (1968); and W. T. Thomson in "Theory of Vibration with Applications", Prentice Hall, Englewood Cliffs, N.J. (1988). Such a damper is an auxiliary device for dissipating energy in the form of heat by means of the frictional resistance between the primary structure (the rotating mechanical structure to be damped) and a supplemental mass. In general, its effectiveness is lower than that of comparable dynamic absorbers.
Conventional torsional absorbers are described by H. Frahm in U.S. Pat. No. 989,958 granted in 1911, J. Ormondroyd and J. P. Den Hartog in "The Theory of the Dynamic Vibration Absorber", Transactions of ASME, Vol. 50, p.2 (1928); W. K. Wilson, supra; and Thomson, supra. Such an absorber is an auxiliary vibratory system which modifies the vibration characteristics of the primary structure by providing a supplemental mass which is elastically connected to the primary structure by a metallic or a rubber spring assembly. Since the natural frequency of the absorber is constant, it can be effective only when the disturbance frequency coincides with the frequency to which the assembly is tuned. Furthermore, when the conventional absorber is used to deal with a given resonant condition, its effect is to replace the troublesome resonant peak by two new resonant conditions, one below and the other above the original resonant peak.
Centrifugal pendulum absorbers have been described by B. C. Carter in U. K. Patent No. 337,466 in 1929, J. P. Den Hartog in "Tuned Pendulums as Torsional Vibration Eliminators", Stephen Timoshenko 60th Anniversary volume, Macmillan, London, Great Britain (1938); W. K. Wilson, supra; and W. T. Thomson, supra. These also use an auxiliary vibratory arrangement having dynamic behavior which is controlled by a centrifugal acceleration field. The natural frequency of the centrifugal pendulum absorber is directly proportional to the angular velocity of the primary structure and the proportionality constant is fixed as a property of the particular design. Therefore, in order to maintain the highest possible absorption efficiency against the disturbance frequencies, the ratio between the disturbance frequency and the angular velocity of the primary structure should remain constant.
All of these passive vibration suppression devices fail to function effectively when the frequency of the disturbance varies freely in time. However, a series of active vibration absorption systems have been presented in the literature by Y. Furuishi and J. Taketou in "Vibration Control of Structure by Active Mass Damper", Transactions of JSME, Vol. 52, No. 474c, p. 683 (1986); Yoshida et al. 1988, K. Seto and Y. Furuishi in "A Study on Active Dynamic Absorber", Proceedings of ASME, Modal Analysis, Modeling, Diagnostics and Control. DE-38, p. 263 (1991); H. Nagato and Y. Yoshida in "Control of Active Dynamic Vibration Absorber by Neural Network", Transactions of JSME, Vol. 58, No. 550, p. 1755 (1992).
The concept of a delayed resonator (DR) has been recently described by N. Olgac and B. Holm-Hansen in "A Novel Active Vibration Absorption Technique: Delayed Resonator", Journal of Sound and Vibration, Vol 176, p. 93 (1994); N. Olgac in U.S. Pat. No. 5,431,261 granted Jul. 11, 1995, and N. olgac in U.S. Pat. No. 5,505,282 dated Apr. 9, 1996. In such a device, a simple position feedback is used with a time delay to tune the absorber to have it "sensitive" to a given frequency. Utilizing this control, a conventional absorber setting is forced to mimic an ideal resonator. The DR absorber can remove the primary oscillations completely within a certain range around the natural frequency of the mass-spring-damper setting used. However, although its principles are effective to deal with vibrations of a translational character, it is not effective in dealing with torsional vibrations in which the frequency of disturbance varies with the angular velocity of the primary structure such as in the crankshaft and transmission system of aircraft, automotive and marine propulsion engines.
It is an object of the present invention to provide a novel absorber for damping the vibrations of varying frequency in a rotating structure.
It is also an object to provide such an absorber which may be readily coupled to various rotating structures to dynamically damp varying frequencies of vibration.
Another object is to provide a novel vibration damping assembly which will damp both vibrations resulting from variations in rotational torque and vibrations of a translational nature.
A further object is to provide a novel method for damping the vibrations in a rotating structure resulting from external disturbance torques.
Yet another object is to provide a novel method for damping both vibrations of a translational nature and those resulting from rotational torque.